Many aircraft are designed to transport cargo instead of, or in addition to, passengers. The cargo is stored in unit load devices (ULDs) within a cargo bay of the aircraft. The ULDs are typically filled with cargo outside of the aircraft, moved into the cargo bay and then moved to a desired position within the cargo bay for storage during transport. The ULDs are supported in the interior cargo bay by a plurality of rollers providing a low friction transport plane which is commonly referred to as a conveyor plane. The cargo bay can include many power drive units (PDUs) that each have one or more tires for interfacing with and transporting the cargo laterally (i.e., in a direction parallel to the wingspan of the aircraft) or longitudinally forward and aft. The cargo bay also has two separate areas in relation to ULD motion. One area laterally aligned with the cargo door has an omni-directional function to allow for lateral and longitudinal translation. The area of the cargo system either forward or aft of the cargo door only requires longitudinal translation. Thus, traditional aircraft include interior PDUs that only move the cargo forward and aft and orthogonally oriented PDUs laterally aligned with the door to the cargo bay that can move cargo laterally and forward/aft. In order to change the direction of cargo movement, one set of parallel oriented PDUs can be lowered below the conveyor plane, and then the orthogonally oriented PDUs adjacent to them can be raised above the conveyor plane.
Traditional PDUs utilize an induction motor to rotate the tire as well as to raise and lower the PDU relative to the conveyor plane. In response to power being removed from the PDU, the motor of the PDU generates no torque and is lowered below the conveyor plane. In response to the PDU being lowered, any cargo above the PDU can move relative to the conveyor plane as the conveyor plane may provide relatively little friction. This is undesirable as unsecured cargo can become damaged, can damage other cargo and/or can change the weight distribution of the aircraft.